Plesiosaurs are large reptiles characterized by long necks with small heads; stocky, finless tails; and a broad, turtle-like body with four flippered appendages. Based on their morphological design and fossilized feces[1], paleontologists infer that they were sea-dwelling carnivores that dined on fish and shellfish.

Their fossil skeletons were first discovered in early 19th century in England by Mary Anning and were among the first large vertebrate fossils found. Since then, many more have been found and there are a number of complete specimens.

The name plesiosaur (or plesiosaurus) comes from the Greek for "near to lizard", and was named by William Conybeare to signify that it was more lizard-like than the fish-like ichthyosaurus, which was first found only a few years previously nearby.

According to evolutionary scientists, who use relative dating methods that rely on the strata fossils are found in, plesiosaurs lived in the Jurassic period before becoming extinct. According to creation scientists, who use the Bible, plesiosaurs were created on the fifth day of the creation week[2] and lived concurrently with Man. It is also widely held among creation researchers that the leviathan mentioned in the Bible[3] was a type of plesiosaur, specifically the kronosaurus[4].

Like other aquatic animals, according to the Bible plesiosaurs were not taken aboard the Ark during the Great Flood[5]. Because of this, some creation researchers believe that not all of them perished in the Flood and that populations survive to this day[2], hidden beneath the icy waters of seas and deep lakes. The most often cited evidences for this theory are the many modern sightings around the globe of creatures that fit the description of a plesiosaur, the most famous of which have been reported at Loch Ness, Scotland (the so-called "Loch Ness Monster" or "Nessie") and Lake Okanagan in British Columbia (known as "Ogopogo").

There was also a reputed plesiosaur carcass[2] dredged up by the Japanese fishing trawler Zuiyo Maru near New Zealand in 1977, but many researchers believe it was the remains of a basking shark[6][7][8].